Vent holes are often provided in housings of various devices including: automotive electrical/electronic components such as headlamps, rear lamps, fog lamps, turn lamps, motors, various pressure sensors, and pressure switches; cameras; videos; information terminals such as mobile phones; electric shavers; electric toothbrushes; and lamps for outdoor use. The main purpose of providing a vent hole in a housing of a device is to allow the interior of the device to communicate with the exterior thereof so as to avoid an excessive increase in the pressure inside the housing of the device associated with an increase in the temperature therein during operation of the device. A vent hole is also provided in a casing of a battery for the purpose of discharging a gas generated during operation of the battery.
In order to prevent entry of water or dust through a vent hole provided in a housing of a device, an air-permeable filter is placed over the vent hole in some cases. It is common practice to attach the above air-permeable filter to the housing of the device using an adhesive layer such as a pressure-sensitive adhesive tape because the use of the adhesive layer is an easy and reliable way to attach the filter to the housing at low cost.
A porous membrane made of a polyolefin resin or a fluorine resin is often used as the above air-permeable filter. In particular, a porous membrane obtained by stretching polytetrafluoroethylene (hereinafter referred to as “PTFE”) to form a microporous structure therein (hereinafter referred to as a “stretched porous PTFE membrane”) is known as an air-permeable filter excellent in water repellency. However, sebum, a surfactant, oil, or the like may contact an air-permeable filter depending on the environment in which the filter is used. Even if a stretched porous PTFE membrane excellent in water repellency is used as an air-permeable filter, entry of a liquid having a low surface tension cannot be fully prevented. Accordingly, depending on the intended use, an air-permeable filter is subjected to oil-repellent treatment using a treating agent containing a fluorine-containing polymer.
It is well known that a fluorine-containing polymer having a linear perfluoroalkyl group (hereinafter, a “linear perfluoroalkyl group” may be referred to as an “Rf group”) having 8 or more carbon atoms is suitable for imparting oil repellency. Rf groups having 8 or more carbon atoms have a significantly higher crystallinity than Rf groups having a smaller number of (for example, 6 or less) carbon atoms. This high crystallinity is considered to contribute to the development of excellent oil repellency. It is also known that, due to its high crystallinity, a treating agent having an Rf group having 8 or more carbon atoms exhibits a large receding contact angle, (which is one of the dynamic contact angles and another of which is an advancing contact angle). The receding contact angle increases with increasing crystallinity, and sharply increases as the number of carbon atoms increases from 6 to 8. For these reasons, it is common practice to use a treating agent containing a fluorine-containing polymer having an Rf group having 8 or more carbon atoms to impart oil repellency to an air-permeable filter.
It is also known that oil repellency is imparted to an air-permeable filter using another treating agent together with the above-mentioned treating agent. For example, Patent Literature 1 discloses treating an air-permeable filter with a treating agent that contains a fluorine-containing polymer having an Rf group and that also contains a fluorine resin having a fluorine-containing alicyclic structure in its main chain (claim 1, etc.). The fluorine resin having a fluorine-containing alicyclic structure has excellent film formation properties, and can be obtained, for example, by polymerization of perfluoro(2,2-dimethyl-1,3-dioxole) (paragraphs [0009] and [0011]). Patent Literature 1 teaches that the number of carbon atoms in the perfluoroalkyl group is 4 to 16, and particularly preferably 6 to 12 (paragraph [0023]). However, in Examples, a fluorine-containing polymer having perfluoroalkyl groups having 9 carbon atoms on average is used according to the common practice described above (paragraphs [0049] and [0050]; and Examples).